Composite wire insulation has been made by coextruding melt-fabricable fluoropolymer coating onto wire, which coating is foamed as it leaves the extruder, and a solid fluoropolymer skin onto the foaming coating, such as disclosed in U.S. Pat. Nos. 4,711,811 and 4,716,073. The presence of the foaming during the extrusion and the coming together of the coextruded fluoropolymers makes it difficult to control the thicknesses of the two layers forming the wire insulation, which is especially important as the thickness of these two layers is decreased to provide smaller diameter insulated wire. In addition, the adhesion between the layers (foam and skin) suffers from the foaming action occurring as the adhesion between the layers is forming.
Foamed tubing, i.e. tubing with a foamed sidewall, is desirable for forming more flexible and/or lightweight tubing, but it is very difficult to make foamed tubing because there is no center wire to support the tubing during solidification from the molten extrudate. Moreover, the foaming of the molten extrudate reduces melt strength, furthering the tendency of the tubing to sag and deform, especially as the wall thickness and/or diameter of the tubing increases.
There is a need for the ability to form foamed fluoropolymer structures which do not have the foregoing problems.